Photochemistry of oxygenated and deoxygenated solutions of the photosensitizer (2E,5E)-2,5-bis(4-dimethylaminobenzylidene)-cyclopentanone, a ketocyanine dye

Article abstract of DOI:10.1016/j.molstruc.2015.10.056

Photochemical studies of (2E,5E)-2,5-bis(4-dimethylaminobenzylidene)-cyclopentanone (I) have been carried out. Compound I has been shown to sensitize the production of singlet oxygen (¹O₂) in deuterated toluene (C₇D_8<

Full text of DOI:10.1016/j.molstruc.2015.10.056

Journal of Molecular Structure 1105 (2016) 396e402
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Photochemistry of oxygenated and deoxygenated solutions of the
photosensitizer (2E,5E)-2,5-bis(4-dimethylaminobenzylidene)-
cyclopentanone, a ketocyanine dye
*
Christopher A. Zoto , Mine G. Ucak-Astarlioglu, Robert E. Connors
Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, MA 01609, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 August 2015
Received in revised form
Photochemical studies of (2E,5E)-2,5-bis(4-dimethylaminobenzylidene)-cyclopentanone (I) have been
1
carried out. Compound I has been shown to sensitize the production of singlet oxygen ( O
2
) in
deuterated toluene (C
used as the indicator for
to an investigation of the chemical reactivity of I with
spectroscopy, and HPLC/MS experimental data, it is proposed that the self-sensitized photooxidation of I
7
D
8
), chloroform (CDCl
3
), and methanol (CD
3
OD). Tetramethylethylene (TME) was
3
October 2015
1
O
2
production. Photobleaching of solutions of the dye was observed, which led
Accepted 19 October 2015
Available online 23 October 2015
1
O
2
. Based on 1H NMR, UVeVisible absorption
1
with
pentadione as products. Photooxidation of I was not detected in CH
absence of photooxidation in CH OH is attributed to the short lifetime of singlet oxygen in this solvent
s), compared to its lifetime in CD OD ( s). UVeVisible absorption spectra of irra-
O
2
yields 4-dimethylaminobenzaldehyde and (E)-3-(4-dimethylaminobenzylidene)-1,2-cyclo-
Keywords:
3
OH but was found in CD OD. The
3
Singlet oxygen
Photosensitizer
Photooxygenation
Photoisomerization
3
(
t
D
¼ 9.5
m
3
t
D
¼ 270
m
diated solutions of I in deoxygenated solvents demonstrate that (E,E) / (E,Z) photoisomerization of the
dye takes place under these conditions.
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
the doubly degenerate
and *, contain two spin unpaired electrons. The first excited
singlet state of oxygen ( O
above the ground state triplet, contains two spin paired electrons in
the * orbital [16].
Singlet oxygen is known to be a reactive intermediate in the
x
p2p* molecular orbitals, designated as p *
p
y
1
ꢀ1
ꢀ1
Substituted and unsubstituted 2,5-diarylidene cyclopentanones
are a class of ketocyanine dyes that have received attention for a
variety of applications, including their use as fluorescent solvent
polarity probes, fluoroionophores, nonlinear optical materials, and
their use as photosensitizers for electronic energy transfer [1e13].
This study was initiated to test and evaluate (2E,5E)-2,5-bis(4-
2
), lying 22.4 kcal mol (93.7 kJ mol )
p
x
photooxidation of various types of organic compounds [17]. For
example, the Schenck ene reaction is a reaction between an olefinic
1
dimethylaminobenzylidene)-cyclopentanone I (Fig. 1) as a photo-
sensitizer for the production of singlet oxygen ( O
(ene) system and O
2
, to form an allylic hydroperoxide [18] along
1
2
). During the
with a concerted shift of the double bond. The Schenck reaction
course of this study, unanticipated results led to an investigation of
the photochemistry of I, the results of which are reported here. The
between 2,3-dimethyl-2-butene (tetramethylethylene, TME) and
1
O
2
, giving 3-hydroperoxy-2,3-dimethyl-1-butene is shown in
þ 2 ] cycloaddition is shown in
first excited electronic state of oxygen, known as singlet oxygen
Fig. 2 (a) [19]. An example of a [2
Fig. 2 (b), where (E)-stilbene reacts with O in the presence of 9,10-
p
p
1
1
(
O
2
), is a strong oxidant that has found widespread applications;
2
consequently there is an ongoing search for efficient and stable
sensitizers for singlet oxygen. Applications of singlet oxygen
include photodynamic therapy (PDT), water purification, and
dicyanoanthracene (DCA) in acetonitrile to give (1) benzaldehyde,
(2) (Z)-stilbene, (3) trans-2,3-diphenyloxirane, and (4) benzil as the
observed photoproducts [20].
chemical/biological decontamination [14,15].
Triplet photosensitization is a common method for producing
singlet oxygen [16]. As shown in Fig. 3, the reaction pathway for the
photosensitized production of singlet oxygen consists of (i)
3
Ground state oxygen ( O
2
) has a triplet spin multiplicity, in that
1
photoexcitation of a ground state photosensitizer ( PS) to a higher
1
*
Corresponding author.
energy excited singlet state ( PS*), followed by (ii) rapid
E-mail address: czoto@hotmail.com (C.A. Zoto).
http://dx.doi.org/10.1016/j.molstruc.2015.10.056
022-2860/© 2015 Elsevier B.V. All rights reserved.
0

C.A. Zoto et al. / Journal of Molecular Structure 1105 (2016) 396e402
397
irradiation. Doroshenko and Pivovarenko [23] have found that the
structurally fixed julolidine analogue of I, (2E,5E)-2,5-
bis((1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9-yl)methy-
lene)-cyclopentanone, undergoes (E,E) / (E,Z) photoisomerization
via its lowest triplet state when irradiated in toluene.
2. Experimental
Compound I was synthesized by the intermolecular base cata-
lyzed crossed aldol condensation of cyclopentanone (1 mol equiv-
alent) with 4-dimethylaminobenzaldehyde (2 mol equivalents) in
the presence of sodium hydroxide. Recrystallization from toluene
yielded pure product, as determined by thin layer chromatography,
which showed one spot upon development. The structure of I was
1
13
firmly established by H, C NMR and IR spectroscopy [24].
All solutions were freshly prepared and all solvents were spec-
tral grade and used as received. Room temperature absorption
®
spectra were measured with a Perkin Elmer Lambda 35 UV/VIS
Fig. 1. Structures of compounds I and II.
7 8 3
spectrometer using toluene (C H ), chloroform (CHCl ) or meth-
1
3
anol (CH OH) solvents. H NMR spectra were obtained using a
Bruker® AVANCE 400 MHz NMR spectrometer. For LC/MS tests, an
intersystem crossing from the singlet to the triplet excited state
3
®
manifold ( PS*), whereupon (iii) collisional interaction of the
Agilent Technologies 1200 system equipped with a C-18 nonpolar
3
®
lowest triplet excited state ( PS*) with triplet (ground) state oxygen
reverse phase column, and an Agilent Technologies 6130 Quad-
1
results in energy transfer, yielding
O
2
. One of the challenges in
rupole spectrometer was employed. Ammonium formate was used
as the buffer for both the organic and aqueous eluting solvents. All
solutions were filtered through cotton prior to running LC/MS
measurements.
evaluating useful and robust photosensitizers of singlet oxygen is to
1
determine their resistance to oxygenation by the O
2
that they have
produced (self-sensitized photooxygenation).
Previous research has shown that compounds similar to I un-
dergo (E,E) / (E,Z) photoisomerization. The photoisomerization of
Irradiation studies were carried out using a 150 W Xenon arc
lamp and
a
Newport® glass
filter
(green:
cut-on
(
2E,5E)-2,5-dibenzylidene-cyclopentanone (II) (Fig. 1), the unsub-
wavelength ¼ 400 nm). Simultaneous with irradiation, samples
stituted analogue of I, has previously been studied by George and
Roth [21] and Ucak-Astarlioglu [22]. It was found that this com-
pound undergoes photoisomerization in tetrahydrofuran (THF) and
that (E,E) $ (E,Z) photoequilibrium is established after 45 min of
had a continuous stream of either O gas (for photooxidation ex-
2
periments) or N2 gas (for deoxygenated experiments) bubbled
through them.
TME was used as the indicator for the generation of singlet
2
p O in the presence of
] cycloaddition reaction where (E)-stilbene reacts with 1
Fig. 2. Examples of characteristic reactions of singlet oxygen: (a) A Schenck ene reaction. (b) A [2
p
þ 2
9,10-dicyanoanthracene in acetonitrile to give (1) benzaldehyde, (2) (Z)-stilbene, (3) trans-2,3-diphenyloxirane, and (4) benzil as photoproducts.

398
C.A. Zoto et al. / Journal of Molecular Structure 1105 (2016) 396e402
Fig. 3. Photosensitized production of singlet oxygen.
3
oxygen in the photooxidation experiments. As noted earlier, TME
reacts with singlet oxygen by way of the Schenck reaction, giving 3-
hydroperoxy-2,3-dimethyl-1-butene as the product. It has been
shown that NMR detection of this product is a simple and effective
irradiated while having a fine stream of O
2
bubbled through them.
at different irra-
bubbling intervals were recorded over 225 min. The
NMR peaks of I at t ¼ 0 min decrease with respect to increased
irradiation/O bubbling. Emergence over time of signal peaks at
9.68 (s,1H), 8.52 (s), 7.68 (d, 2H), 7.58 (d), 6.65 (d), 3.02 (s, 6H),
1
H NMR spectral data of a solution of I in CDCl
3
diation/O
2
method to test for singlet oxygen production [19]. TME has one
2
1
singlet at
d
ca 1.6 ppm (12H). Fig. 4 provides the H NMR assign-
d
ments of 3-hydroperoxy-2,3-dimethyl-1-butene [25]. The hydro-
peroxy H atom labeled ‘e’ is observed only in CDCl
2.993 (s), and 2.990 (s) ppm was observed. The emergence of the
resonance at 9.68 ppm suggests the generation of an aldehyde.
Recalling that singlet oxygen undergoes the Schenck reaction with
olefins and that I contains olefinic bonds, a two step ene reaction
that leads to the production of 4-dimethylaminobenzaldehyde as a
product is proposed and shown in Fig. 5. The mechanism for the
3
.
3
. Results and discussion
1
3
.1. Testing of I as a photosensitizer for O
2
photooxidation reaction involves the [2
p
þ 2
p] cycloaddition of
1
1
ground state I with O , producing the corresponding endoper-
Testing of I as a photosensitizer for O
2
was carried out in C
7
D
8
,
2
oxide as an intermediate, which due to thermal instability, breaks
CDCl , and CD OD solvents. The NMR spectra of a catalytic amount
3
3
down into 4-dimethylaminobenzaldehyde and (E)-3-(4-
dimethylaminobenzylidene)-1,2-cyclopentadione. The proposed
mechanism for the photooxidation of I is supported by previously
of I irradiated in the presence of TME in each of the deuterated
solvents shows conversion of TME to the allylic hydroperoxide (3-
hydroperoxy-2,3-dimethyl-1-butene). Identity of the hydroperox-
ide was confirmed by experimental NMR data that is in excellent
agreement with literature values shown in Fig. 4. Thus it is
confirmed that I is a photosensitizer for the production of singlet
oxygen in different types of solvents. Supplementary Information
provides NMR spectra in all three deuterated solvents.
published work on 1,2-cycloaddition reactions of olefinic systems
1
with
2
O , followed by cleavage of endoperoxide intermediates.
1
2
Murthy et al. [26] have demonstrated that light generated O re-
acts with substituted olefins in a 1,2-cycloaddition reaction,
generating an endoperoxide intermediate that breaks down into
two carbonyl compounds. In addition, Cabrerizo, et al. [27] have
studied reactions of heteroaromatic compounds, including multi-
3.2. Self-sensitized photooxidation of I
1
substituted pyrazines which react with O
2
, yielding endoperoxides
that break down into dicarbonyl compounds.
During the course of experiments using I to generate singlet
1
The H NMR spectrum of I in CDCl
irradiation/O bubbling was compared to the H NMR spectrum of
commercially available 4-dimethylaminobenzaldehyde (99%, Sig-
3
solution after 225 min of
oxygen in various solvents, photofading of solutions was observed.
Photofading occurred in the absence of substrates, which suggests
that self-sensitized photooxidation of I could be taking place. A
series of experiments using several deuterated and undeuterated
solvents were performed where solutions of I were simultaneously
1
2
1
3
maeAldrich) in CDCl and peaks consistently matched. The H NMR
spectrum of 4-dimethylaminobenzaldehyde consists of a singlet at
Fig. 4. 1H NMR assignments of 3-hydroperoxy-2,3-dimethyl-1-butene.

C.A. Zoto et al. / Journal of Molecular Structure 1105 (2016) 396e402
399
Fig. 5. Proposed two step reaction scheme for the photooxidation of I.
3
d
6
d
9.77 ppm (1H, aldehyde proton), two doublets at
.64 ppm (2H), which are aromatic protons and a singlet at
3.01 ppm (6H, eN(CH protons).
Guided by the proposed reaction scheme, the 1H NMR signals
that emerge at 8.52 (s), 7.58 (d), 2.993 (s), and 2.990 (s) ppm are
d
7.68 (2H) and
From this result, it is concluded that I does not react with O
significant degree over 40 min.
2
to any
3
)
2
Interestingly, as shown in Fig. 6 (c), irradiation/O
2
bubbling
experiments for I in CH OH showed no change in the absorption
spectrum over the course of 120 min. This result was a bit sur-
prising in that it appears to be in conflict with the H NMR data for I
which clearly showed 4-dimethylaminobenzaldehyde and the
3
d
1
assigned to the dione. Integrations were carried out on the emerged
1
H NMR signals and it was found that calibrating the singlet at
d
8.52 ppm, believed to be the vinyl H, to 1 the doublet at
integrates to approximately 4 (aromatic doublets), and the closely
spaced peaks at 2.993 and 2.990 ppm integrate to approximately
and 6 (2 ꢁ CH and eN(CH protons), which is consistent with
d
7.58 ppm
3
proposed dione as self-sensitized photoproducts in CD OD. The key
1
to understanding the apparent disagreement between the H NMR
and UVeVisible results is recognition of the fact that the NMR ex-
periments were performed with totally deuterated solvents and the
irradiations for UVeVisible analysis were performed using
undeuterated solvents.
d
4
2
3 2
)
the chemical structure of the dione. Further support for the pro-
posed reaction scheme is provided by LC/MS measurements which
showed signals with m/z ratios of 150 and 230, which are the M þ 1
values of the aldehyde and proposed dione photoproducts.
In addition to examining the self-sensitized photooxidation of I
It is known [28] that the lifetime of singlet oxygen is longer in
deuterated solvents than in their undeuterated form (see Table 1).
1
The difference is particularly dramatic for methanol in which
undergoes ~40ꢁ increase in lifetime (
reported value of 270 s in CD OD [29] compared to a relatively
short 9.5 s in CH OH [30].
The knowledge that the lifetime of singlet oxygen is significantly
longer in CD OD than in CH OH suggested using UVeVisible
spectroscopy to examine the photochemistry of I in CD OD under
2. The same concentration of I and similar irradiation intervals
O
2
1
using H NMR spectroscopy, photooxidation experiments of I were
monitored using UVeVisible absorption spectroscopy by collecting
D
t ) upon deuteration, with a
m
3
absorption spectra at different irradiation/O
series of UVeVisible absorption spectra of I in (a) C
and (c) CH OH at different irradiation/O bubbling intervals is
shown in Fig. 6. It is observed that the long wavelength absorption
band (S /S ) for I in C and CHCl decreases with respect to
increased irradiation/O bubbling time. Concurrently, emergence of
two higher energy spectral bands is observed. The highest energy
absorption band centered at ~330 nm conforms well to the ab-
sorption spectrum of 4-dimethylaminobenzaldehyde (shown by
the dashed line in each solvent). It is proposed that the second
emerged spectral band (lower in energy) 380e420 nm represents
absorption of the alkylamino substituted 1,2-cyclopentadione. The
location of this spectral band is in agreement with the conjugation
path length of this compound, intermediate between 4-
2
bubbling intervals. A
m
3
7
H
8
(b) CHCl
3
3
2
3
3
3
0
1
7
H
8
3
O
2
3
were used as in the study conducted in CH OH. It is shown from
Fig. 6(d) that photooxidation of I occurs in CD
products as in undeuterated CHCl and C
3
OD yielding the same
l
3
H
7 8
solvents. We propose
1
that the increased reactivity observed for I with
compared to CH
I and the lifetime of O
long enough, photooxidation of I is observed.
2 3
O in CD OD
3
OH is in general a function of the concentration of
1
1
2
2
in the solvent used. If the lifetime of O is
3.3. (E,E) / (E,Z) photoisomerization of I
dimethylaminobenzaldehyde and I.
1
To confirm that I is reacting with singlet (excited state) O
2
and
3
In their photoisomerization studies of the julolidine analogue of
I, Doroshenko and Pivovarenko [23] observed photochemistry in
. The dominant absorption band at approximately 465 nm,
not triplet (ground state) O
a solution of I in C , which involved wrapping the solution with
aluminum foil and bubbling the sample with O . It was shown that
essentially no changes were observed in the absorption spectrum
of I in C for the dark experiment between 0 min and 40 min.
2
, a dark experiment was carried out for
7 8
H
7 8
C H
2
representing the (E,E)-photoisomer, was reduced and a higher en-
ergy spectral band at approximately 350 nm increased in intensity
with prolonged irradiation, arising in a band difference of 115 nm.
7 8
H

400
C.A. Zoto et al. / Journal of Molecular Structure 1105 (2016) 396e402
1
.2
1
UVeVisible absorption spectroscopy.
Similar characteristic light induced changes were observed in
the absorption spectra of I in deoxygenated (a) C , (b) CHCl , and
(c) CH OH. As shown in Fig. 7, the decrease in intensity of the strong
long wavelength absorption band of the starting (E,E) isomer of I
and growth of a shorter wavelength band in the UV region is
observed with an isosbestic point in between. The new band is
7
H
8
3
0
0
0
0
.8
.6
.4
.2
3
(a)
(b)
7 8
assigned to absorption by the (E,Z)-isomer of I (Fig. 7). In C H , the
dominant absorption band at 450 nm, representing the (E,E)-iso-
mer of I, was reduced and a higher energy spectral band at 330 nm
increased in intensity with prolonged irradiation, arising in a band
difference of 120 nm. The wavelength maxima locations of the
0
.4
.2
1
1
1
(E,E)- and (E,Z)-isomers of I, along with the spectral band difference
are consistent with the published data for its julolidine analogue
0
0
0
0
.8
.6
.4
.2
0
[23]. It was demonstrated in these studies that I undergoes (E,E) /
(E,Z) photoisomerization in deoxygenated solutions (reaction
scheme is shown in Fig. 8).
Photochemical experiments for (2E,5E)-2,5-dibenzylidene
cyclopentanone (II, Fig. 1), the unsubstituted analog of I, have
1
been reported. George and Roth used H NMR and IR spectroscopy
0
0
0
0
.8
.6
.4
.2
0
1
.2
1
(c)
0
.8
0.6
(a)
0
0
.4
.2
0
0
0
.8
.4
0
(d)
1
0
0
.8
.6
3
00
400
500
600
Wavelength (nm)
(
b)
Fig. 6. UVeVisible absorption spectra of I after various irradiation/O
tervals in (a) C
([I] ¼ 0.015 mM): t ¼ 0, 1, 2, 3, 5, 7, 10, 16, 20, 25, and 40 min; (b)
CDCl OH
([I] ¼ 0.022 mM): t ¼ 0, 2, 4, 6, 8, 11, 14, 20, 30, and 60 min; (c) CH
OD ([I] ¼ 0.014 mM): t ¼ 0,
, 10, 20, 30, 40, 50, 60, 75, 90, 105, and 120 min. The absorbance spectrum of
2
bubbling in-
7
H
8
0.4
3
3
(
5
[I] ¼ 0.012 mM: after 120 min with 5e10 intervals; (d) CD
3
0
.2
0
commercially available 4-dimethylaminobenzaldehyde is shown by the dashed line in
each solvent.
0
.8
The emerged spectral band was assigned to absorption by the (E,Z)-
photoisomer of the compound. The appearance of an isosbestic
point between initial and product bands occurs when the molar
extinction coefficients of both isomers are equal and is a charac-
0.6
(c)
teristic feature of the (E,E) / (E,Z) photoisomerization of
saturated ketones [31]. We have studied the photochemistry of I in
deoxygenated solvent environments (bubbled with N ) using
2
a,b-un-
0
0
.4
.2
0
Table 1
Singlet oxygen lifetimes (
vents [29,30].
D
t ) in various sol-
3
00
400
500
600
Wavelength (nm)
Solvent
D
t (ms)
CHCl
CDCl
3
229
7000
9.5
Fig. 7. UVeVisible absorption spectra of I after various irradiation/N
tervals in (a) C
([I] ¼ 0.015 mM): t ¼ 0, 10, 30, 60, 90, 120, 150, 180, 210, and
40 min; (b) CDCl OH
([I] ¼ 0.017 mM): t ¼ 0, 25, 45, 65, 85, 105, and 135 min; (c) CH
[I] ¼ 0.013 mM): t ¼ 0, 10, 40, 70, 100, 130, 160, and 190 min.
2
bubbling in-
3
7 8
H
CH
CD
3
OH
OD
2
(
3
3
3
270

C.A. Zoto et al. / Journal of Molecular Structure 1105 (2016) 396e402
401
at
grows in at ~300 nm. The weak absorption that is observed
400e500 nm is due to excitation to S which has been shown
through theory and experiment to be n/ * for II [32]. Given the
l
¼ 350 nm. An isosbestic point occurs at ~310 nm and a band
~
1
p
well-studied and understood photochemistry of II, the similar
spectroscopic behavior of I and II following irradiation of deoxy-
genated solutions supports the assignment of the changes observed
for I to (E,E) / (E,Z) photoisomerization.
Fig. 8. (E,E) / (E,Z) photoisomerization of I.
A reaction scheme for the photochemistry of I is shown in
Fig. 10. Photoexcitation of I promotes the molecule to the first
excited singlet state, where it undergoes intersystem crossing to
the underlying triplet manifold at a rate that depends upon the
solvent. From the lowest excited triplet state, photooxidation and
1
.2
1
(
E,E) / (E,Z) photoisomerization are competing photochemical
3
*
3
reactions. For photooxidation, I reacts with O
2
, affording ground
, which can react with each other to produce the
photooxidized products reported here. In the special case of CH OH
where no reaction was observed, the low value implies a high
intersystem crossing and, as shown with the dark
1
state I and
O
2
0
0
0
0
.8
.6
.4
.2
0
3
t
D
1
3
rate of O
2 2
/ O
3
experiment of I in C
7 8 2
D , there is no reaction between I and O . In
the absence of oxygen, only (E,E) / E,Z) photoisomerization is
observed for I.
4. Conclusion
Compound I has been shown to sensitize the production of
singlet oxygen through its reaction with TME to give 3-
hydroperoxy-2,3-dimethyl-1-butene in C 8, CDCl , and CD OD.
The observation of photobleaching of solutions led to a study of the
photochemical reactivity of I in the presence and absence of O
7
D
3
3
2
.
3
00
400
500
600
1
1
From H NMR studies, it is concluded that I reacts with
þ 2 ] cycloaddition reaction, giving an endoperoxide inter-
mediate, which breaks down into 4-dimethylaminobenzaldehyde
and (E)-3-(4-dimethylaminobenzylidene)-1,2-cyclopentadione.
2
O in a
Wavelength (nm)
[2p
p
Fig. 9. UVeVisible absorption spectra of II in deoxygenated THF at t ¼ 2, 3, 5, 7, 9, 11,
and 13 min of irradiation.
LC/MS analysis of the photooxidized material confirmed 4-
dimethylaminobenzaldehyde (m/z ¼ 150) and a component with
m/z ¼ 230, which is M þ 1 of the proposed dione. Interestingly,
to show that the (E,Z) photoisomer is the major product when (E,E)-
II is irradiated in THF solvent [21]. Our laboratory has also irradiated
photooxidation of I was not observed in CH OH. The failure to
3
(
E,E)-II in deoxygenated THF. HPLC reveals that a single photo-
detect photooxidation in CH OH is attributed to the short lifetime
3
1
product signal is produced which [22], in agreement with the
previous workers, we assign to the (E,Z) photoisomer of II. Since the
photochemistry is uncomplicated and the identity of the single
photoproduct is known, a series of UVeVisible absorption spectra
of II in deoxygenated THF were recorded at various irradiation in-
tervals (Fig. 9) to compare with the photochemical results for I in
similarly deoxygenated solvents. The spectra show that the
maximum absorption intensity for the (E,E) isomer of II is reduced
of O2 in CH OH compared to longer lifetimes in other solvents,
3
including CD OD.
3
Irradiation of deoxygenated solutions of C H CHCl , and CH OH
7
8,
3
3
indicate (E,E) / (E,Z) photoisomerization of I. With increased
irradiation time, the decrease in intensity of the first strong ab-
sorption band (assigned to the (E,E) isomer), growth of a higher
energy band (ca
l 330 nm) (assigned to the (E,Z) isomer), and an
isosbestic point in between is consistent with (E,E) / (E,Z)
Fig. 10. Reaction diagram showing the competition between photooxidation and photoisomerization of I.

402
C.A. Zoto et al. / Journal of Molecular Structure 1105 (2016) 396e402
photoisomerization. Further, comparative studies with the known
photoisomerization of its symmetric unsubstituted analogue, II,
show similar spectral behavior, thereby providing additional
experimental evidence for the (E,E) / (E,Z) photoisomerization of I
in deoxygenated solutions.
[9] W. Tao, W. Fei-peng, S. Men-quan, Chem. Res. Chin. Univ. 19 (2003) 470.
10] M.V. Barnabus, A. Liu, A.D. Trifunac, V.V. Krongauz, C.T. Chang, J. Phys. Chem.
[
[
96 (1992) 212.
11] T.J. Dougherty, C.J. Gomer, B.W. Henderson, G. Jori, O. Kessel, M. Korbelik,
J. Moan, Q. Peng, J. Natl. Cancer. Inst. 90 (1998) 889.
[
12] J. Xue, Y. Zhao, F. Wu, D.-C. Fang, J. Phys. Chem. A 114 (2010) 5171.
13] (a) K.W. Woodburn, C.J. Engelman, M.S. Blumenkranz, Retina 22 (2002) 391;
[
(b) S.B. Brown, E.A. Brown, I. Walker, Lancet. Oncol. 5 (2004) 497.
Acknowledgments
[14] M.C. DeRosa, R.J. Crutchley, Coord. Chem. Rev. 233e234 (2002) 351.
[
[
15] C. Changtong, D.W. Carney, L. Luo, C.A. Zoto, J.L. Lombardi, R.E. Connors,
J. Photochem. Photobiol. A Chem. 260 (2013) 9.
16] N.J. Turro, Modern Molecular Photochemistry, University Science Books,
Sausalito, CA, 1991.
During his almost 40 year teaching career at Worcester Poly-
technic Institute, Dr. Connors was an inspirational and strongly
devoted educator and mentor to all his students. He enjoyed
research and teaching undergraduate and graduate students in the
area of physical chemistry. This manuscript is dedicated to the late
Dr. Robert E. Connors.
[
[
17] H.H. Wassermann, J.L. Ives, Tetrahedron 37 (1981) 1825.
18] (a) G.O. Schenck, Naturwissenschaften 35 (1948) 28;
(b) H.H. Wassermann, R.W. Murray, Singlet Oxygen, Academic Press, New
York, 1979.
[
[
19] M. Matsumoto, M. Yamada, N. Watanabe, Chem. Commun. 6 (2005) 483.
20] Z. Yuan, S. Zheng, Y. Zeng, J. Chen, Y. Han, Y. Li, Y. Li, New J. Chem. 3411 (2010)
7
18.
Appendix A. Supplementary data
[21] H. George, R.J. Roth, Tetrahedron Lett. 43 (1971) 4057.
[
22] M.G. Ucak-Astarlioglu, The Electronic Structure and Spectroscopy of Diary-
lidene- Cycloalkanones and Their Protonated Cations, Ph. D. dissertation,
Worcester Polytechnic Institute, 2003.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.molstruc.2015.10.056.
[
23] A.O. Doroshenko, V.G. Pivovarenko, J. Photochem. Photobiol. A Chem. 156
(
2003) 55.
24] C.A. Zoto, Structural and Photophysical Properties of Internal Charge Transfer
Arylidene and 2,5-Diarylidene Cyclopentanones, Ph. D. dissertation,
[
References
2-
Worcester Polytechnic Institute, 2012.
[
[
[
1] V.G. Pivovarenko, A.V. Klueva, A.O. Doroshenko, A.P. Demchenko, Chem. Phys.
Lett. 325 (2000) 3918.
2] P.K. Das, R. Pramanik, D. Banerjee, S. Bachi, Spectrochim. Acta A 56 (2000)
[
[
[
25] R.W. Murray, W. Kong, S.N. Rajadhyaksha, J. Org. Chem. 58 (1993) 315.
26] R.S. Murthy, M. Bio, Y. You, Tetrahedron Lett. 50 (2009) 1041.
27] F.M. Cabrerizo, M.L. D ꢀa ntola, G. Petroselli, A.L. Capparelli, Photochem. Pho-
2763.
tobiol. 83 (2007) 526.
3] A.O. Doroshenko, A.V. Grigorivich, E.A. Posokhov, V.G. Pivovarenko,
A.P. Demchenko, Mol. Eng. 8 (1999) 199.
4] J. Kawamata, K. Inoue, T. Inabe, Bull. Chem. Soc. Jpn. 71 (1998) 2777.
5] P. Kaatz, D.P. Shelton, J. Chem. Phys. 105 (1996) 3918.
6] J. Kawamata, K. Inoue, T. Inabe, M. Kiguchi, M. Kato, Y. Taniguchi, Chem. Phys.
Lett. 249 (1996) 29.
[
[
28] P.B. Merkel, R. Nilsson, D.R. Kearns, J. Am. Chem. Soc. 94 (1972) 1030.
29] S. Jockusch, R.X. Ren, Y.P. Jang, Y. Itagaki, H.R. Vollmer-Snarr, J.R. Sparrow,
K. Nakanishi, N.J. Turro, J. Am. Chem. Soc. 126 (2004) 4646.
30] C. Schweitzer, R. Schmidt, Chem. Rev. 103 (2003) 1685.
31] S. Zhang, S. Liu, Q. Zhang, Y. Deng, Chem. Commun. 47 (2011) 6641.
32] R.E. Connors, M.G. Ucak-Astarlioglu, J. Phys. Chem. A 107 (2003) 7684.
[
[
[
[
[
[
[
[
7] J. Kawamata, M. Akiba, Y. Inagaki, Jpn. J. Appl. Phys. 42 (2003) L17.
8] J. Wu, M. Shi, Y. Zhao, F. Wu, Dyes & Pigments 76 (2008) 690.